Though you've been here a week I haven't said it so: Welcome!

About size: if you want to maximize the size of your world map to be able later to zoom in and retain a lot of pixels, try using a simpler style for the overall world map. The more layers and gradients and details, the fatter your file will be for a given set of dimensions. One solution might be to generate a raw world map at an insanely high resolution, without detailing it. save that as a master that you can clip put regions from, and finish doing your world detailing on a reduced-side copy. For instance do put cities on the biggest world map- at least main ones -- just leave them as simple dots. Only make pretty icons for then ( if your style calls for that) with a somewhat cut-down version destined to be your detailed world map. Do indicate rivers on the biggest, but maybe not stroked with a nice taper. Do put borders on the biggest version, just not symbolized or color-rimmed.

About this world map you're showing us early stages of... Where does that fit on a globe? Most projections are going to be wider than they are tall. Decide on a reasonable projection for your main world map - based on the arrangement of land masses one might suffer less distortion than another-- or at least less awkwardly placed distortion. When you do generate a reasonable world, don't clip the view so close to the land masses. Some small-scale maps you're going to want plenty of ocean shown, whether to focus on ocean characteristics, or just as a convenient no interfering place to put legend info. Just search go the word "projection" and read the first 25 threads that mention it. Some will have good tips - particularly commentary by Hai Etlik, who is one of our resident advocates for plausible geodesy and projection choice.

You *could* call what you already have a world map, if that world is mostly water that you aren't showing; also if what is shown only goes to mid-latitudes. Hmmm - in other words if it isn't a comprehensive world map but essentially a regional one ;-).